The Failure Drill, when practiced and perfected, is an extremely useful drill that saves lives.
The modern Failure Drill is accomplished by the shooter firing two center-mass shots as quickly as those shots can be delivered into a target and within an 8-inch center mass area. Following the second shot, the shooter follows through and assesses if their target still presents a deadly threat. If the target is still a threat, then the third shot is taken and ideally hits the area between the eyebrows and the upper lip. Note: The shot is aimed at this area of the skull because this is where the bone is thinnest.
The Failure Drill is more difficult than it sounds.
The most difficult part of the Failure Drill is the changes of shot delivery that the shooter must make. The first two shots should be fired as quickly as the shooter can center his hits. Then, they must slow down enough to re-evaluate the situation and with precision deliver a headshot.
The history behind this technique (and something to practice): The Mozambique Drill: A History and How To